Polymer lead covering with varied material properties

ABSTRACT

This document discusses, among other things, a lead assembly including a lead body, at least one conductor extending through the lead body, and a covering having varied material properties. In an example, the covering is made by forming pieces of material having varied material properties. In another example, the covering is made by varying parameters such as heat or tension during wrapping of a piece of material onto a lead assembly.

TECHNICAL FIELD

This patent document pertains generally to medical device leadassemblies, and more particularly, but not by way of limitation, topolymer lead coverings having varied material properties.

BACKGROUND

Medical devices such as pacers and defibrillators typically include atleast one lead assembly. In a defibrillator, for example, a leadassembly typically includes at least one defibrillation electrode, suchas a defibrillation coil. Some lead assemblies include a cover thatextends over at least a portion of the outer surface of the leadassembly. A cover may extend over a defibrillation coil, for example.Covers are used, for example, to prevent tissue ingrowth. Some coversare formed from polytetrafluoroethylene (PTFE). Improved coverings forlead assemblies are needed.

SUMMARY

An example lead assembly includes a lead body for a medical device leadassembly, at least one conductor extending through the lead body, afirst cover formed from a polymer and extending over a first portion ofthe lead body, and a second cover formed from the polymer and extendingover a second portion of the lead body. The first cover has a first setof material properties. The second cover has a second set of properties.At least one of the second set of material properties is different fromthe first set of properties. In an example, the first cover includes afirst sheath and the second cover includes a second sheath. In anexample, the first sheath is joined to the second sheath. In an example,the lead assembly further includes a polymer strip helically wrappedaround the lead body, the polymer strip forming the first cover and thesecond cover. In an example, a first portion of the polymer stripforming the first cover is wrapped at a first tension, and a secondportion of the polymer strip forming the second cover is wrapped at asecond tension that is different than the first tension. In anotherexample, the ultra-high molecular weigh polyethylene lead assemblyincludes a polymer coating on the lead body, the coating including thefirst cover and the second cover. In an example, the first cover isporous and the second cover is non-porous. In an example, a leadassembly includes a drug disposed in at least some of the pores in thefirst cover. In an example, the polymer is polytetrafluoroethylene(PTFE) or ultra-high molecular weigh polyethylene. The polymer isoptionally expanded. In an example, the second cover includes anexpanded polymer and a filler disposed in pores in the second cover. Inan example, the filler is silicone or polyurethane.

In another example, a lead assembly includes a lead body for a medicaldevice lead assembly, at least one conductor extending through the leadbody, at least one electrode, and a cover extending over at least aportion of the lead body. The cover is formed from a polymer. A firstportion of the cover has a first set of properties, and a second portionof the cover has a second set of properties. At least one of the secondset of properties different from at least one of the first set ofproperties. In an example, the first portion of the cover has a firstdielectric strength and the second portion of the cover has a seconddielectric strength that is different than the first dielectricstrength.

An example method includes forming a cover of polymer having a porosity,applying the cover to a medical device lead assembly, and selectivelyvarying the porosity of the polymer at locations on the cover. In anexample, applying the cover includes wrapping a strip of polymermaterial on the medical device lead assembly under tension, andselectively varying the porosity of the polymer at locations on thecover includes selectively controlling the tension during wrapping ofthe strip of polymer material. In an example, wrapping the strip ofpolymer material on the medical device lead assembly includes wrapping afirst portion of the cover at a first tension and wrapping a secondportion at a second tension lower than the first tension, whereinwrapping the first portion at the first tension creates larger poresthan wrapping the second portion at the second tension. In an example,selectively varying the porosity of the polymer at locations on thecover includes applying a filler to a first portion of the cover, thefiller filling at least a portion of a multiplicity of pores in thefirst portion of the cover. In an example, selectively varying theporosity of the polymer at locations on the cover includes applying heatto the polymer.

Another approach includes forming a first cover formed from an expandedfirst polymer, forming a second cover from a nonexpanded first polymer,applying the first and second covers to a medical device lead assembly,and joining the first cover to the second cover. In an example, applyingthe first and second covers includes wrapping respective first andsecond strips of polymer material around the medical device leadassembly. In an example, the method further includes applying a thirdcover formed from the polymer to the medical device lead assembly, thethird cover formed from an expanded polymer, and joining the third coverto the second cover.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which are not necessarily drawn to scale, like numeralsdescribe substantially similar components throughout the several views.The drawings illustrate generally, by way of example, but not by way oflimitation, various embodiments discussed in the present document.

FIG. 1 shows an example system for monitoring and stimulating a heartincluding a lead having a covering with varied material properties.

FIG. 2A shows a medical device lead assembly that includes a coverhaving varied material properties.

FIG. 2B shows a medical device lead assembly that includes a coverextending having varied material properties extending over a portion ofa lead.

FIGS. 3A and 3B show a piece of polymer material wrapped around aportion of a medical device lead assembly to form a cover.

FIG. 4 shows a piece of polymer material wrapped spirally around aportion of a medical device lead assembly to form a cover.

FIG. 5 shows a porous cover having pores of different sizes in differentregions of the cover.

FIGS. 6 and 7 are flow charts that illustrate a method of applyingpolymer material to a lead assembly.

DETAILED DESCRIPTION

The following detailed description includes references to theaccompanying drawings, which form a part of the detailed description.The drawings show, by way of illustration, specific embodiments in whichthe invention may be practiced. These embodiments are also referred toherein as “examples.” The drawings and following detailed description isnot to be taken in a limiting sense, and the scope of the presentinvention is defined by the appended claims and their equivalents.

A medical device lead assembly includes a polymer covering extendingover at least a portion of the lead assembly and having variedproperties at locations on the lead assembly. In an example,performance, reliability, and/or manufacturability are enhanced byvarying material properties in the polymer cover. The varied propertiesin the polymer covering can be achieved by using two or more coversegments that are made from polymer materials having differentproperties (e.g. different porosities), by controlling parameters suchas tension and heat during application of a polymer material to a leadassembly, and/or by varying the material properties after the polymercovering is applied to the lead assembly. Varied properties can also beproduced by forming a piece of polymer having varying properties, andthen applying the polymer to a lead assembly. In an example, propertiesare imparted to portions of the lead based upon the performancerequirement of specific portions of the lead. In an example, propertiesof a lead are supplied so that electrical conductivity is provided nearelectrodes, tissue ingrowth and drug loading is supported at a fixationpoint such as at distal tip, and/or knick-resistance is provided whereneeded, such as near a terminal.

Forming a cover from a single polymer having varying material propertiescan provide a number of advantages. In an example, a covering havingvarying material properties provides desirable handling characteristics,such as flexibility, pushability, and/or abrasion resistance. Varyingforms (e.g. expanded and non-expanded polytetrafluoroethylene (PTFE)) ofa single polymer tend to bond well, while bonds between dissimilarmaterials (e.g. polyethylene and PTFE) can provide weak points in a leadassembly. In an example, varying the properties in lead assemblycovering limits fibrotic adhesion and tissue ingrowth and supportsextractability of a lead. In an example, a portion of a lead islaser-markable, thereby eliminating the need for a label.

FIG. 1 shows an example system for monitoring and stimulating a heartthat includes at least one lead assembly including a covering havingvaried material properties at locations on the lead assembly. FIGS. 2,3A-3B, 4, 5, and 6A-6B show lead assemblies and example coverings. FIGS.6 and 7 are flowcharts that illustrate methods of producing leadassemblies having varying material properties at locations on acovering.

Referring now to FIG. 1, an example system for monitoring andstimulating a heart 105 includes a medical device 110 and at least onelead assembly including at least one covering having varied materialproperties at varied locations on the covering. In an example, the leadassembly is a pacing lead, defibrillation lead, or neurological lead. Inan example, the lead assembly delivers an antitachyarrhythmia therapy,bradycardia therapy, heart failure therapy, or neural stimulation, forexample. In an example, the medical device 110 is a pacer,defibrillator, or stimulator.

In an example, two or more lead assemblies are coupled to the medicaldevice. In FIG. 1, one lead assembly 115 extends into the right side ofthe heart. The other lead assembly 120 extends into the left side of theheart. In another example, the medical device 110 is coupled to a singlelead assembly that extends, for example, into either the right or leftside of the heart. In other examples, a lead assembly extends on oraround the heart, or on or around a nerve truck or other anatomicaltarget.

Referring again to FIG. 1, each lead assembly includes at least onecovering 125, 130 having varied material properties at varied locationson the lead assembly. In an example, a covering includes two or morecovers, such as sheaths, that have different material properties. In anexample, the covers are formed from the same polymer, but have differentmaterial properties, such as different stiffnesses or porosities. In anexample, the two covers are joined together. In an example, a portion135 of a cover 130 that is inserted through the coronary sinus 150during implant is more flexible than a portion 140 of the cover 130 thatis not inserted through the coronary sinus. In another example, aportion 155 of a cover near a distal end 160 of a lead assembly includespores sized to support tissue ingrowth, and other portions of the coverare not porous or includes pores sized to inhibit tissue ingrowth.

FIG. 2A shows another example medical device lead assembly 200. The leadassembly 200 includes one or more conductors extending through a lumenin a lead body 205, and a covering 210 extending over the lead body. Inan example, the lead body 205 is made of silicone. In an example, thelead body is a tube that includes at least one lumen. The covering 210has varied material properties at locations on the lead assembly. Aproximal end 215 of the lead assembly 205 is connectable to a medicaldevice. A distal end 220 of the lead assembly is implantable in, on, oraround a heart. The conductors in the lead assembly are electricallycoupled to one or more electrodes. In an example, the lead assemblyincludes a first defibrillation electrode 230, a second defibrillationelectrode 235, and a sensing/pacing electrode 225. The covering is shownpartially cut-away in FIG. 2A to show the defibrillation electrodes 230,235 beneath the covering.

Referring again to FIG. 2A, for the purpose of illustration, examplelocations of the covering having particular material properties areindicated with a dotted line in FIG. 2A. In an example, a first locationon the covering has a first set of properties including porosity,dielectric strength, knick-resistance, strength, and/or lubricity, forexample, and a second location on the covering has a second set ofproperties, at least one of which is quantifiably different from thefirst set. In an example, at a location 240 near the distal end 220 ofthe lead, the covering 210 has a porosity that allows tissue ingrowth tofixate the distal end of the lead. At locations 245, 250 near thedefibrillation electrodes 230, 235, the covering 210 has a porosity thatsupports conduction of electricity through the cover. In an example, anantitachyarrhythmia therapy is effectively delivered through a coverhaving a porosity that supports electrical conduction. In an example,the pores in the cover at locations near the defibrillation electrodesare large enough to support conductivity, but small enough to preventtissue ingrowth. In an example, at a location 256 on the covering 210that extends through particularly complex vasculature when implanted,the cover has an increased flexibility. In an example, other portions ofthe cover 257, 258 have enhanced knick-resistance and/or abrasionresistance to improve the durability of the cover. In an example, thecovering 210 is formed by wrapping a piece of polymer material onto thelead assembly, and the material property variations are pre-manufacturedinto the piece of material or are created as the polymer material iswrapped onto the lead assembly.

In an example, the lead assembly covering is formed frompolytetrafluoroethylene (PTFE) and expanded polytetrafluoroethylene(ePTFE). In an example, the ends of a lead assembly include ePTFE thatallows tissue ingrowth to fixate the ends of the lead, and otherportions of the lead include less porous or fully dense PTFE. Fullydense PTFE provides good abrasion resistance, dielectric integrity, andlubricity. In an example, portions of a covering near or over anelectrode, such as a defibrillation electrode, are formed from ePTFE toprovide electrical conductivity through the covering, and portions neara terminal are formed from fully dense PTFE to provide knick-resistance.

In other examples, a covering is formed from polyethylene (PE),polypropylene (PP), fluorinated ethylene propylene (FEP),ethylene-tetrafluoroethylene (ETFE), or another biocompatible polymer.In an example, at least part of a cover includes expanded polymer.Expanded polymers bond well with a non-expanded version of the samepolymer, but have different material properties.

Referring now to FIG. 2B, in another example, a covering 260 includesseparate covers 265, 270, 275 having different material properties. Inan example, the covers 265, 270, 275 are made from different forms (e.g.varying porosity) of the same polymer. In an example, the covers 265,270, 275 are joined together, for example by heat bonding, adhesivebonding, welding, solvent bonding, or laser sintering. Covers 265, 275extend over defibrillation electrodes 230, 235 and a portion of a leadbody 205. In an example, at least a portion of the covers 265, 275extending over the electrodes 230, 235 are conductive. In an example,the covers 265, 275 have pores that fill with body fluid, which conductselectricity. Cover 270 extends between the other two covers 265, 275. Inan example, cover 270 is not porous, and has good abrasion resistance.In an example, another cover 280 near the distal end 220 has pores sizedto allow tissue ingrowth.

In an example, a cover is formed by extruding a polymer into a tube orother lead body shape. In another example, a cover is formed by coatingsome or all of a lead assembly with a polymer, for example by spraycoating or dip coating a lead assembly.

In another example, a cover is made by wrapping a piece of polymermaterial (e.g. polymer tape) around a lead assembly, as illustrated byFIGS. 3A and 3B. A piece of polymer material 305 is wrapped around atleast a portion of a lead assembly 315. The piece includes a first edge325 and a second edge 330. The first edge 325 meets or overlaps with thesecond edge 330, as shown in FIG. 3B. In an example, the piece ofpolymer material 305 is wrapped around an electrode 310 that includes,for example a wire 320 wrapped into a coil. In an example, the material305 forms a generally tubular cover 335. In an example, the polymermaterial 305 extends beyond the electrode. In an example, the material305 covers most or all of the lead assembly.

The polymer material 305 is secured on the lead assembly, for example,by joining the cover to itself. In an example, at least a portion of thepolymer material 305 is heated to fuse the polymer material to itself.In an example, the polymer material 305 is sintered proximate the firstedge 325. In an example, the polymer cover 335 is sintered with a laser,infrared (IR) wand, heat gun, or oven. In an example, the heating of thematerial also conforms the polymer material to the outer shape of theelectrode or lead body. In an example, the material properties in aportion of the cover are modified by heating the portion of the cover.

In an example, a second piece of polymer material 340 is wrapped on thelead 315 to form a second cover 345 adjacent to the first cover 335. Inan example, the second cover 345 is formed from the same material as thefirst cover 335, and optionally has different material properties. In anexample, the first and second pieces 305, 340 are wrapped with differenttensions, and/or at different temperatures, to impart different materialproperties to the polymer. In an example, the second cover 345 is joinedto the first cover 335, for example by laser sintering. In an example,the pieces of polymer material 305, 340 include pores at the edges, inwhich an adhesive is disposed. The adhesive supports bonding of thepieces of material to each other or to themselves at locations ofmaterial overlap.

Referring now to FIG. 4, another method of applying a polymer materialto a lead assembly is shown. A strip 405 of polymer material 406 iswrapped in a spiral around at least a portion lead assembly 415. In anexample, a first edge 420 of the strip 405 meets or overlaps with asecond edge 425 of the piece from a previous wrap around the leadassembly. The spiral-wrapped piece of polyethylene forms a cover 430.

In an example, a second piece 440 of polymer material is wrapped aroundthe lead assembly to form a second cover 445 adjacent to the firstcover. In an example, the first cover 430 extends over the electrode 410and has a porosity that supports conductivity through the cover, and thesecond cover has no pores, or a lower porosity than the first cover, toprovide desired durability. In an example, the second cover 445 isjoined to the first cover 430, for example by laser sintering. In anexample, the second cover is formed from the same material as the firstcover, and the second cover has material properties that are differentthan the first cover. In an example, the first cover is formed fromePTFE and the second cover is formed from PTFE.

In an example, the material properties vary within the cover 410. In anexample, some or all of the cover 410 is porous. In an example, all ofthe lead assembly is covered except a fixation mechanism at or near thedistal tip, and a terminal that connects to a medical device. In anexample, the strip of material is manufacture with regions of that havevarying material properties before application to the lead assembly.When the strip material is wrapped on the lead assembly, the cover hasregions having varied properties determined at least in part by thevariations that are manufactured into the raw polymer material. In anexample, the location on the lead assembly of regions having particularproperties can be calculated using the diameter of the lead body, thewidth of the strip, and the stretching characteristics of the polymer.

In another example, the material properties are varied in the cover 430by controlling parameters such as heat and tension in the polymer duringthe wrapping process. In another example, the material properties of thecover are modified after the cover is applied, for example by heating aportion of the cover. In some examples, a combination of these fourtechniques (separate covers, pre-manufactured variations in the strip,controlling application parameters, and post-application processing) isused to achieve a cover having the desired properties.

Referring now to FIG. 5, an example polymer covering 505 includes pores510. The size of the pores is exaggerated for the purpose ofillustration. In an example, the pore size in the polymer covering iscontrolled to control tissue ingrowth into the covering. In an example,the pores 515 in a first portion 520 of the polymer covering 505 arelarger than the pores 525 in a second portion 530 of the polymercovering. For the purpose of illustration, a dotted line is providedFIG. 5 to distinguish the first portion 520 of the covering from thesecond portion 530. In an example, the pores 515 in the first portion520 are large enough to allow at least some tissue ingrowth, and thepores 525 in the second portion 530 are small enough to substantiallyinhibit tissue ingrowth. In an example, pores are provided in the firstportion to support conductivity, and other portion(s) of the coveringinclude no pores to electrical insulate a portion of the lead. In anexample, the tissue ingrowth into the pores 515 in the first portion 520secures the lead to body tissue. In an example, the covering 505 isformed around the lead assembly using the technique illustrated in FIGS.3A-3B or the technique illustrated in FIG. 4. In an example, thecovering 505 includes two adjacent covers fused together. In anotherexample, the covering 505 includes a single cover having materialproperties that vary at locations in the cover.

The size of pores in the polymer material can be controlled using one ormore of a variety of techniques. In an example, a piece of polymermaterial is manufactured to have differing pore sizes by controllingparameters such as tension or heat during the manufacturing process. Inan example, the first portion 520 and second portion 530 of the covering505 of FIG. 5 are formed from separate pieces of polymer material.

In another example, pore size is controlled by adjusting tension appliedto the polymer material as the material is assembled onto the leadassembly. In an example, a polymer cover is made using a spiral windingtechnique, as illustrated in FIG. 4, and the pore size is controlled byvarying the tension on the piece of material 405. In another example,pore size is varied through application of heat during or after theapplication of the polymer material to the electrode. In an example,heating the polymer melts the polymer and/or shrinks the size of thepores. In another example, a filler is applied to porous polymer to fillin pores and change the porosity of the polymer material at a locationon a cover. In an example, the polymer is ePTFE or polyethylene. In anexample, the filler is silicone or polyurethane. In an example, thefiller is treated to provide better adhesion to such polymers. In anexample, the filler is applied to a covering in solution using sprayingor dipping techniques. In an example, two or more of the precedingtechniques are used concurrently or sequentially to control the poresize at one or more locations in the polymer material.

In an example, when the pores are filled with a conductive substance,such as body fluid, the pores in the polymer provide a conductivepathway for a defibrillation current. In another example, the polymerincludes particles of conductive matter to make the covering itselfconductive. In another example, a conductive material is deposited onthe polymer to provide a conductive pathway for a defibrillationcurrent.

In an example, a polymer cover is hydrophilicly treated. In an example,a drug is disposed in at least some of the pores of the first cover. Inan example, a drug is disposed in pores in a portion of a cover near adistal end of the lead assembly.

FIG. 6 is a flow chart that illustrates a method. At 605, a cover isformed having a porosity. At 610, the cover is applied to a medicaldevice lead assembly. In an example, the cover is formed using one ofthe techniques shown in FIGS. 3 and 4. In another example, the cover isformed by extrusion. At 615, the porosity of the polymer is varied atlocations on the cover. In an example, the porosity is varied by varyingthe porosity in the polymer before it is applied to the lead assembly.In another example, the porosity is varied as the polymer is applied tothe lead assembly, for example by controlling application of heat and/ortension during the application of the polymer to the lead assembly. Inanother example, the porosity is varied after the polymer is applied tothe lead assembly, for example by filling pores at locations on thelead.

FIG. 7 illustrates another method. At 705, a first cover is formed froma polymer. In an example, the cover is formed from an expanded polymer.At 710, a second cover is formed from the same polymer. In an example,the second cover is formed from a non-expanded form of the polymer. Inan example, the second cover is formed from ePTFE and the first cover isformed from ePTFE. At 715, the first and second covers are applied to amedical device lead assembly. In an example, the covers are formed in aflat form (e.g. a strip of material), and are then wound onto themedical device lead assembly, for example as shown in FIG. 3 or 4. Inanother example, the covers are extruded into a tube or other shape andthen assembled onto a medical device lead assembly. At 720, the firstcover is joined to the second cover, for example by laser sintering oradhesive bonding. At 725, a third cover is applied to the lead assembly.The third cover is formed from the expanded polymer. At 730, the thirdcover is joined to the second cover. In an example, the first and thirdcovers extend over first and second defibrillation electrodes. Inanother example, the first cover is located at or near distal tip andincludes pores sized to support tissue ingrowth.

Polymer lead coverings are also described in copending application Ser.No. 11/150,549 to Aron et al., entitled “Lead Assembly with PorousPolyethylene Cover”, which is incorporated by reference in its entirety.

It is to be understood that the above description is intended to beillustrative, and not restrictive. Since many embodiments of theinvention can be made without departing from the scope of the invention,the invention resides in the claims hereinafter appended.

1. A method comprising: forming a first cover formed from an expandedfirst polymer; forming a second cover from a nonexpanded first polymer;applying the first and second covers to different longitudinal portionsof a medical device lead assembly; and joining a portion of the firstcover to a portion of the second cover.
 2. The method of claim 1,further comprising selectively varying the porosity of at least one ofthe first cover or the second cover along a portion thereof.
 3. Themethod of claim 2, wherein selectively varying the porosity of the firstcover or the second cover includes applying a filler to at least aportion of one or more pores in the cover.
 4. The lead assembly of claim3, wherein the filler is silicone.
 5. The lead assembly of claim 3,wherein the filler is polyurethane.
 6. The method of claim 2, whereinselectively varying the porosity of the first cover or the second coverincludes applying heat to the cover.
 7. The method of claim 1, whereinapplying the first and second covers includes wrapping a first strip ofpolymer material around a first longitudinal portion of the medicaldevice lead assembly and a second strip of polymer material around asecond longitudinal portion of the medical device lead assembly.
 8. Thelead assembly of claim 7, wherein wrapping the first strip of polymermaterial and the second strip of polymer material around the medicaldevice lead assembly includes wrapping the first strip of polymermaterial at a first temperature and wrapping the second strip of polymermaterial at a second temperature that is different than the firsttemperature.
 9. The method of claim 7, wherein wrapping the first stripof polymer material and the second strip of polymer material around themedical device lead assembly includes wrapping the first strip ofpolymer material at a first tension and wrapping the second strip ofpolymer material at a second tension that is lower than the firsttension.
 10. The method of claim 1, further comprising: applying a thirdcover formed from the expanded first polymer to a third longitudinalportion of the medical device lead assembly; and joining a portion ofthe third cover to a portion of the second cover.
 11. The method ofclaim 1, wherein the first cover is applied proximate a distal end ofthe lead assembly, the third cover is applied proximate a proximal endof the lead assembly, and the second cover longitudinally extendsbetween the first and third cover, the first and third covers includingtissue ingrowth pores.
 12. A lead assembly comprising: a longitudinallyextending lead body for a medical device lead assembly; at least oneconductor extending through the lead body; a porous first cover and adrug disposed in at least one of the pores in the first cover, the firstcover formed from a polymer and substantially extending over a firstlongitudinal portion of the lead body near a distal end of the leadbody, the first cover having a first set of material properties; anon-porous second cover formed from the polymer and substantiallyextending over a second longitudinal portion of the lead body differentthan the first longitudinal portion, the second cover having a secondset of properties, at least one of the second set of material propertiesdifferent from the first set of properties; and a porous third cover anda drug disposed in at least one of the pores in the third cover, thethird cover substantially extending over a third longitudinal portion ofthe lead body near the proximal end of the lead body; wherein at leastone of the first cover or the second cover has greater flexibility thanthe third cover.
 13. The lead assembly of claim 12, wherein the firstcover includes a first sheath and the second cover includes a secondsheath.
 14. The lead assembly of claim 13, wherein the first sheath isjoined to the second sheath.
 15. The lead assembly of claim 12, whereinthe polymer is polytetrafluoroethylene (PTFE).
 16. The lead assembly ofclaim 15, wherein the first cover includes expanded PTFE.
 17. The leadassembly of claim 12, wherein the polymer is ultra-high molecular weightpolyethylene (UHMW PE).
 18. The lead assembly of claim 17, wherein thefirst cover includes expanded ultra-high molecular weight polyethylene.19. The lead assembly of claim 12, wherein the third cover includes anexpanded polymer.
 20. The lead assembly of claim 12, wherein the firstcover includes a first polymer strip substantially wrapped around thefirst longitudinal portion of the lead body, and the second coverincludes a second polymer strip substantially wrapped around the secondlongitudinal portion of the lead body different from the firstlongitudinal portion, an end of the first polymer strip contacting anadjacent end of the second polymer strip.
 21. A lead assemblycomprising: a lead body for a medical device lead assembly; at least oneconductor extending through the lead body; at least one electrode; and acovering including a first polymer strip wrapped around a first portionof the lead body and a second polymer strip wrapped around a secondportion of the lead body, the first polymer strip having a first set ofproperties and the second polymer strip having a second set ofproperties, at least one of the second set of properties different fromat least one of the first set of properties; wherein the first polymerstrip is wrapped around the lead body at a first tension and the secondpolymer strip is wrapped around the lead body at a second tension thatis different than the first tension.
 22. The lead assembly of claim 21,wherein an end of the first polymer strip contacts an adjacent end ofthe second polymer strip.
 23. The lead assembly of claim 21, wherein thefirst polymer strip has a first porosity and the second polymer striphas a second porosity that is different than the first porosity.
 24. Thelead assembly of claim 21, wherein the first polymer strip includesexpanded polytetrafluoroethylene and the second polymer strip includesnon-expanded polytetrafluoroethylene.
 25. The lead assembly of claim 21,wherein the first polymer strip includes expanded polyethylene and thesecond polymer strip includes non-expanded polyethylene.